It is well known in the art to produce positive photoresist formulations from alkali-soluble novolak resins together with light sensitive materials (photoactive compounds), usually a substituted naphthoquinone diazide compound which acts as dissolution rate inhibitor with respect to the resin, dissolved in an organic solvent or mixture of solvents. These photoresist formulations are applied as a thin film or coating to a substrate suitable for a particular application. Upon exposure of selected areas of the coated substrate to actinic radiation, the dissolution rate inhibitor or photoactive sensitizer causes the exposed areas of the coating to be more soluble in developer than the unexposed areas. Conversely, in those areas of the coated substrate not exposed to radiation, the photoactive compound inhibits dissolution of the photoresist film by the developer. Immersing the exposed substrate in alkaline developing solution causes the exposed areas of the photoresist coating to be dissolved while the unexposed areas are largely unaffected. Thus, a positive relief pattern is formed on the substrate. A similar process is used in lithographic printing plates as well.
Among the properties of a photoresist composition which are important in commercial practice and application are the following: photospeed of the resist, development contrast, resist resolution, thermal stability of the image, process latitude, line width control, clean development, unexposed film loss and the like.
Of greatly increasing importance is the ability of such photoresists to reproduce very small dimensions, for example sub-micron profiles or geometries, for use in electronic devices or components. Similarly important is the ability to produce such sub-micron profiles which can withstand high temperature environments or heat encountered in the device fabrication sequence. In processing steps which generate high heat, harsh plasma metal etching or ion implantation for example, the imaged resist must withstand the elevated temperatures with minimum or no critical dimension (CD) changes, edge rounding, and/or image or profile distortion or flow. Additionally, sidewall profiles must be as close to vertical as possible in order to obtain maximum replication of the masked critical dimension features during plasma etching or ion implantation. It is therefore desirable that the photoresist image be thermally stable at a temperature of 130.degree. C. or above, preferably stable at a temperature of about 140.degree.-150.degree. C. or above, so that plasma etch and ion implantation steps among others can be made harsher and consequently allow faster processing.
It has been known for some time that positive photoresists based on high molecular weight novolak resins and diazonaphthoquinone compounds as sensitizers or photoactive compounds can produce reasonably stable images that can withstand temperatures as high as about 120.degree.-125.degree. C. without rounding or flowing or change of critical dimensions. However, as indicated hereinbefore, the geometries required have decreased dramatically and the fabrication process generates temperatures of 130.degree. C. or higher. Photoresists able to provide photoresist images capable of withstanding such elevated temperatures cannot be easily accomplished solely by utilizing high molecular weight novolak resins. As the molecular weight of the novolaks go up, the resolution capabilities are generally reduced and resolution reduction is incompatible with the demands of high resolution devices wherein the geometries can be as low as 0.5 micron. Moreover, the fractionation of the novolak resins from suitable solvents necessary to obtain such high molecular weight novolaks, by eliminating or reducing the presence of low molecular weight novolak resin fractions, is cumbersome and results in undesirable yield loss and a slowing of photospeeds.
The photoactive compounds used today are generally sulfonate esters of diazonaphthoquinones and hydroxylic aromatics, such as those disclosed in U.S. Pat. Nos. 2,797,213; 3,106,465; 3,130,047; 3,148,983; 3,201,329; 3,785,825 and 3,802,885. It has also been recognized that the thermal resistance capabilities of positive photoresist images are improved as the diazo ester content of the photoresist formulation is increased. This is especially true for photoresist formulations containing esters of hydroxybenzophenones (HBP), e.g. tri- or tetra-hydroxybenzophenones, with diazonaphthoquinones (DNQ), e.g. 1,2-diazonaphthoquinone-4- or -5- sulfonyl chloride. However, as one increases the diazo ester content of the photoresist formulation, the solubility or shelf life of the formulation decreases. Yet, to be commercially viable, the formulation must survive at least one year without precipitation in the container.
Recently in U.S. Pat. No. 4,863,828 the use of 1,2-naphthoquinonediazide-4- or -5- sulfonate esters of 2,3,4,3',4',5'-hexahydroxybenzophenone as photoactive compounds for producing positive photoresist resins of improved thermal stability has been disclosed. However, while these esters of hexahydroxybenzophenone produce positive photoresist images having better thermal stability than those produced from corresponding esters of tri- or tetra-hydroxybenzophenones, the photospeed characteristics of the former are generally not as good as the latter. Furthermore, the esters of the hexahydroxybenzophenones do not permit a wide variation in ratio of novolak resin to photoactive compound without significant changes in photospeed, thermal behavior, contrast or resolution and without any significant loss in unexposed areas of photoresist.
It is therefore an object of this invention to provide novel photoactive compounds for use in providing positive photoresist formulations for producing positive photoresist images having improved thermal stability yet which allow a wide variation in resin to sensitizer ratio without significant changes in photospeed, thermal behavior, contrast or resolution and with virtually no loss in unexposed areas of the photoresist.
Another significant problem which results from high diazo ester content is the phenomenon called outgassing or popping which can occur when the coated photoresist is exposed to UV radiation by using exposure apparatus typically operating in a monochromatic mode, known as G-line, I-line, broad band and others. Typically, during exposure of such photoresist coatings, microscopic bubbles form in or next to the exposed areas. These bubbles result from the evolution of nitrogen, a material by-product of the UV induced reactions resulting from the diazo ester component. As the ratio of diazonaphthoquinone to the ketone in the tri-, tetra- or hexa-hydroxybenzophenone photoactive compound goes up, this phenomenon of outgassing or popping becomes more aggravated. This is particularly true when using esters of tetra- and hexa-hydroxybenzophenones with 1,2-diazonaphthoquinone-4- or -5- sulfonyl chloride, and especially when the ratio of DNQ/HBP is 3.5/1 or higher. Thus, one is confronted with the contradictory requirement of having a diazo-rich ester photoresist formulation to optimize thermal resistance, while trying, at the same time, to minimize or eliminate the phenomenon of outgassing or popping which becomes more severe as the diazo content in the photoresist increases.
It is therefore a further object of this invention to provide high temperature resistant photoresist images from high diazo ester containing positive photoresist compositions which do not suffer from the undesirable phenomenon of outgassing and/or popping during UV exposure.
A still further object of this invention is to provide means for obtaining positive photoresist images having improved thermal stability yet which allow a wide variation in the resin to sensitizer ratio without significant changes in photospeed, thermal behavior, contrast or resolution and with virtually no loss in unexposed areas of the photoresist while, at the same time, providing photoresist formulations wherein the undesirable phenomenon of outgassing or popping during UV exposure, even at high ratios of diazonaphthoquinone/ketone of hydroxybenzophenone, is avoided or substantially eliminated or greatly reduced.